Lithographic printing plates (after process) generally consist of ink-receptive areas (image areas) and ink-repelling areas (non-image areas). During printing operation, an ink is preferentially received in the image areas, not in the non-image areas, and then transferred to the surface of a material upon which the image is to be produced. Commonly the ink is transferred to an intermediate material called printing blanket, which in turn transfers the ink to the surface of the material upon which the image is to be produced.
At the present time, lithographic printing plates (processed) are generally prepared from lithographic printing plate precursors (also commonly called lithographic printing plates) comprising a substrate and a photosensitive coating deposited on the substrate, the substrate and the photosensitive coating having opposite surface properties. The photosensitive coating is usually a photosensitive material, which solubilizes or hardens upon exposure to an actinic radiation, optionally with further post-exposure overall treatment. In positive-working systems, the exposed areas become more soluble and can be developed to reveal the underneath substrate. In negative-working systems, the exposed areas become hardened and the non-exposed areas can be developed to reveal the underneath substrate. Conventionally, the plate is exposed with an ultraviolet light from a lamp through a separate photomask film having predetermined imaging pattern that is placed between the light source and the plate, and the exposed plate is developed with a liquid developer to bare the substrate in the non-hardened or solubilized areas.
Laser sources have been increasingly used to imagewise expose a printing plate that is sensitized to a corresponding laser wavelength, allowing the elimination of the photomask film. Suitable lasers include, for example, infrared lasers (such as laser diode of about 830 nm and NdYAG laser of about 1064 nm), visible lasers (such as frequency-doubled NdYAG laser of about 532 nm and violet laser diode of about 405 nm), and ultraviolet laser (such as ultraviolet laser diode of about 370 nm).
On-press developable lithographic printing plates have been disclosed in the literature. Such plates can be directly mounted on press after imagewise exposure to develop with ink and/or fountain solution during the initial prints and then to print out regular printed sheets. No separate development process before mounting on press is needed, allowing the reduction of labor and the elimination of hazardous waste. Among the patents describing on-press developable lithographic printing plates are U.S. Pat. Nos. 5,258,263, 5,516,620, 5,561,029, 5,616,449, 5,677,110, 5,811,220, 6,014,929, 6,071,675, and 6,482,571.
An on-press developable plate generally comprises on a substrate a photosensitive layer soluble or dispersible in and on-press developable with ink and/or fountain solution. An on-press removable overcoat is often coated over the photosensitive layer to, for example, improve photospeed. Here both the photosensitive layer (in the non-hardened areas) and the overcoat are soluble or dispersible in and on-press removable with ink and/or fountain solution. In order to be able to develop or remove on press with ink and/or fountain solution, the photosensitive layer or the overcoat must be able to be dissolved or dispersed in or softened by ink and/or fountain solution quickly upon contact. Such a photosensitive layer or overcoat generally has limited moisture resistance, limited non-tackiness, limited handling durability, and/or limited block resistance. It would be desirable if a plate and method of using it could be designed which would eliminate the above issues.
The inventor has found that such a desire can be achieved by a lithographic plate comprising on a substrate an ink and/or fountain solution developable photosensitive layer, and a durable overcoat that is removable by brushing or rubbing while in contact with water or an aqueous solution; and by a method of removing the overcoat off press with brushing or rubbing in the presence of water or an aqueous solution, followed by on-press development to remove the non-hardened areas of the photosensitive layer.